1. Field
The present disclosure relates in general to the field of telecommunications and, more specifically to systems and methods for signaling discontinuous transmission (DTX).
2. Description of the Related Technology
Hybrid Automatic Repeat reQuest (HARQ) is used in wireless communications to enable robust transmissions over wireless channels. Transmitted data is first encoded by a code used for error detection, followed by a code used for forward error correction (FEC). Generally, only a subset of the FEC codeword bits is sent in any single transmission attempt.
At the receiver, an attempt is made to correct errors in the received signal through use of the FEC code. If residual errors remain after FEC, the error detection code generates a retransmission request through a negative acknowledgement (NACK). When the transmitter receives a NACK, it may transmit a second (possibly different) subset of codeword bits. The receiver can then combine received data from multiple HARQ transmission attempts in order to increase the probability of a successful decoding. Conversely, if there are no residual errors after FEC, the error detection code informs the transmitter through an acknowledgement (ACK) that no further transmissions are required.
In addition to the cases discussed above, a third outcome called discontinuous transmission (DTX) is also possible. DTX occurs when the control signaling needed for the HARQ process is missed. In this case, the receiver is unaware of the data transmission and in general may not signal either a positive or a negative acknowledgement. However, in the case where the state of multiple parallel HARQ processes is signaled in a single message and control information for at least one of these processes has been received, encompassing DTX information can be explicitly signaled.
On the downlink in LTE, HARQ control information is carried by the Physical Downlink Control Channel (PDCCH), while data is carried by the Physical Downlink Shared Channel (PDSCH). In the Third Generation Partnership Project Long Term Evolution (3GPP-LTE), the three possible outcomes are: 1) DTX, i.e., no control signaling was detected on the PDCCH; 2) ACK, i.e., the user equipment (UE) was able to successfully decode both the control signaling on the PDCCH and the corresponding data transmission on the PDSCH; or 3) NACK, i.e., the UE was able to successfully decode the control signaling on the PDCCH but was not able to decode the corresponding data transmission on the PDSCH.
DTX may occur because no control signaling was sent (this may be referred to as a DTXNoTx event) or because the UE receiver was unable to successfully decode the PDCCH signaling (this may be referred to as a DTXNoRx event). While DTXNoTx is not considered to be an error event, DTXNoRx is considered to be an error event. Although the UE receiver cannot distinguish between DTXNoTx and DTXNoRx events, if the evolved node B (eNB) transmitter knows that a DTX event occurred, then the transmitter can determine whether or not it was a DTXNoTx or DTXNoRx, since the transmitter knows whether or not it transmitted control signaling. Furthermore although the likelihood of DTXNoRx given a physical downlink control channel (PDCCH) was transmitted is dependent on network operation, a nominal value of 1% is often used as a figure of merit.
Being able to differentiate between DTXNoRx and NACK events at the transmitter is useful, since it assists the transmitter in selecting an appropriate power level for the PDCCH and PDSCH channels and in selecting an appropriate subset of codeword bits to send for the next HARQ transmission attempt. This can result in more efficient HARQ operation, less latency introduced by HARQ, and greater overall cell throughput. In addition, knowledge of the occurrence of one or more DTXNoRx events can allow the transmitter to hypothesize that it needs to transmit future HARQ control signaling with more robust encoding and/or transmit power to improve the probability of the receiver being able to successfully decode such control signaling.
In the case of time division duplexing (TDD), the UE can detect a DTXNoRx has occurred because of mismatch between the downlink assignment index (DAI) and the number of received PDCCHs. However, in the case of frequency division duplexing (FDD), the UE may be unaware such an event has occurred as a DAI field is not included in the PDCCH message.
At the receiver side, NACK and DTX events are similar in the aspect that transmitted data cannot be successfully decoded in either case. If data was actually sent, then an HARQ retransmission must be made in both cases if the receiver is to have any chance of decoding the corresponding data block.
From the foregoing, it is apparent that there is a significant need for efficient systems and methods for signaling DTX or for signaling information from which DTX can be concluded. Systems and methods to meet this need are provided in the disclosure discussed herein below.